1. Field of the Invention
The present invention relates to a heat exchanger tube for a falling film evaporator suitable to employ in the falling film evaporator of an absorption refrigeration machine and so forth.
2. Description of the Related Art
In a falling film evaporator employed in absorption water cooling and heating appliance and so forth, a refrigerant flows down along the outer peripheral surface of a heat exchanger tube for performing heat exchanging with a media to be cooled, such as water, flowing through the tube, for cooling the medium. The refrigerant contacting with the heat exchanger tube spreads on the surface of the heat exchanger tube with wetting the latter and evaporates under low pressure to remove heat from a heat transmission surface of the heat exchanger tube to cool the water as the medium to be cooled, in the tube. Upon evaporation of the refrigerant spread on the surface of the heat exchanger tube, vaporization heat is removed from the heat transmission surface so that the water or so forth in the tube can be efficiently cooled. Therefore, in order to attain high performance heat exchanger tube, it is necessary to increase the contact area between the refrigerant and the heat exchanger tube (namely, the area of the heat transmission surface) as great as possible.
Increasing of the contact area between the refrigerant and the heat exchanger tube may be achieved by increasing the surface area of the heat exchanger tube and by enhancing refrigerant spreading ability in spreading of the cooling water with wetting the surface of the heat exchanger tube. As the conventional heat exchanger tube with the increased surface area, there are a flute tube which has grooves formed on the external surface of the tube along the tube axis, and a low fin tube which is provided with collar-like or spiral fin or fins on the external surface of the tube. On the other hand, as the heat exchanger tube having an improved refrigerant wetting and spreading ability, there is a surface treated tube having a smoothed external surface and a surface treated tube having the external surface treated by wire brush polishing. Also, as the heat exchanger tube which can achieve both of the increased external surface area and improved refrigerant wetting and spreading ability, there is proposed a high performance heat exchanger tube, in which cut-outs are formed in the fins arranged on the external surface of the tube in alignment in the tube axis direction (Shuichi Takada "RecentAbsorption Refrigeration Machine and Heat Pump (3)", March, 1989).
However, above-mentioned conventional heat exchanger tubes encounter the following problems. Namely, in the case of the surface treated tube with smoothed or polished external surface, when the refrigerant drops on the surface of the tube, the refrigerant may widely spread with wetting the external surface of the tube in the area near the drop point. However, the refrigerant has a tendency to converge toward the tube axis direction as flowing down along the external surface of the heat exchanger tube to lower wetting and spreading ability. In case of the flute tube, since the refrigerant flows in the tube axis direction along the grooves to achieve higher wetting and spreading ability in comparison with the above-mentioned surface treated tube. However, at ridge portions between the adjacent grooves, no wetting and spreading ability can be obtained. Therefore, the heat transmission area of the whole heat exchanger tube cannot be satisfactorily large. On the other hand, in the case of low fin tube, while the surface area of the external surface of the tube can be increased by the presence of fins arranged on the outer periphery of the tube, the wetting and spreading ability of the refrigerant inherently becomes small since motion of the refrigerant in the tube axis direction is blocked by the fins. Furthermore, though the high performance heat exchanger tube, in which cut-outs are formed in the fins, can achieve certain level of gain in improving the heat exchanging performance, it does not achieve the satisfactorily level of gain of the heat exchanging performance, yet. In the recent years, needs for further higher performance of absorption type water cooling and heating appliance. In order to satisfy such needs, it is strongly desired to have a further improved performance of the high performance heat exchanger tube.